Vacuum pumps are widely used across a broad spectrum of industrial applications including, for example, the manufacture of vacuum coated automotive products, the environmental testing of spacecraft, chemical manufacturing process applications and biological and medical research.
Typical mechanical vacuum pumps have some form of piston which reciprocates or rotates within a cylinder to effect the removal of gas from a chamber or vessel to which the pump intake is connected. Large capacity pumps may have multiple piston-cylinder combinations. The piston or pistons are commonly driven by a shaft mounted on bearings within the vacuum pump. The bearings and pistons must constantly be kept lubricated to avoid problems caused by excessive friction between moving parts such as excessive wear, piston overheating leading to piston seizure and bearing overheating leading to bearing failure. Lubricating oil is commonly used to lubricate the moving parts of the vacuum pump. The lubricating oil also serves a second function in that it forms the necessary gas tight seal between the piston and cylinder ensuring that the gas displaced by the vacuum pump piston(s) does not leak back across the piston-cylinder interface from the high pressure side to the low pressure side of the pump.
A known method for supplying sealing/lubricating oil to shaft bearings and the piston and cylinder is by a combination of gravity feed, centrifugal force and differential pressure. An oil reservoir from which oil flows under gravity is positioned immediately above the pump. Oil lines connect the reservoir to the end caps of the vacuum pump where oil flows into the cylinders through two paths: (1) through the bearings and thence through metering rings; and (2) through oil feed ducts in the vacuum pump side covers. Where a center shaft bearing is provided, oil to one of the end caps is also conducted through a longitudinal axial passage in the shaft connected to an extended port opening onto the bearing.
The natural oil flow from the reservoir is augmented by maintaining the reservoir at atmospheric pressure and conducting the oil into the cylinder of the pump when it is under vacuum and after the oil has lubricated the bearings or other moving parts. Thus, the oil tends to flow through the pump under gravity, the centrifugal force of the rotating shaft and is also forced through the pump by a pressure differential between the reservoir and the pump cylinder. The oil is expelled from the cylinder along with the gas displaced by the vacuum pump piston(s). The gas/oil mixture passes through a separator where the oil is separated from the gas and the oil is returned to the reservoir while the gas is expelled to the atmosphere.
Although this oil distribution system has proved practical under many circumstances, there are conditions wherein the gravity/differential pressure system of lubrication does not supply sufficient oil to effectively lubricate the bearings and seal the piston(s) of the vacuum pump.
As an example, when a vacuum pump is evacuating a relatively large vessel, such as a large vacuum chamber capable of holding a spacecraft for vacuum testing, oil distribution can be a problem. During an initial period of operation of the pump, there will be almost no pressure difference between the cylinder pressure and the atmospheric pressure in the reservoir, thus, there will be no pressure differential augmenting the oil flow from the reservoir. This situation is due to the relatively small volume of the cylinder compared with the much larger chamber volume. Each stroke or rotation of the piston removes a relatively small volume of gas from the chamber, and it may take several minutes, depending upon the chamber size, before any appreciable pressure differential is realized within the chamber and the cylinder. During this initial phase of vacuum pump operation, the oil flows only under gravity and not by any pressure differential. Adequate oil for lubrication and piston/cylinder sealing may not be obtainable under gravity flow alone, leading to increased wear of pump parts and shorter pump life. Catastrophic pump failure could also result in the form of a piston seizure or bearing failure.
Another example is when pressure in the reservoir is reduced by an auxiliary vacuum pump to remove volatile process materials that collect in the oil. If not removed from the oil in the reservoir, the volatile materials re-expand in the cylinders, increasing cylinder pressure above the desired level. The volatiles may also adversely affect the lubricating qualities of the oil. Reduction of reservoir pressure, however, removes the differential pressure component that induces oil flow.
One method of supplying adequate sealing and lubricating oil to vacuum pump pistons and bearings is to use an auxiliary oil pump. Such oil distribution systems are effected by mounting an oil pump on the outside of the vacuum pump housing to pump oil from the reservoir to the bearings and pistons. Such oil pump configurations typically require complicated plumbing, valving and manifolds which drive up the purchase cost of the vacuum pump, as well as the maintenance and operating costs.